This invention relates to methods for generating models of the subsurface characteristics of a well and, more particularly, to a method of generating a subsurface characteristic model which is consistent with readings taken in a borehole with a well logging device.
The subject matter of this invention is related to subject matter disclosed in the following U.S. patent applications, each filed of even date herewith and assigned to the same assignee as the present invention: U.S. application Ser. No. 019,926 of C. Regat; U.S. application Ser. No. 019,925 of W. Kenyon; and U.S. application Ser. No. 019,917 of J. Suau and W. Frawley.
In the drilling of an oil or gas well it is advantageous to obtain as much information as possible about the nature of the formations or beds surrounding the drilled borehole. To this end, well logging devices are lowered into the borehole to measure various characteristic properties of surrounding formations. In electrical logging, for example, a device which includes an electrical sensor is moved through the borehole and provides indications of the electrical resistivity or conductivity of the subsurface materials at different depth levels.
For many well logging applications it is desirable that the logging device respond only to a very limited portion of the formation material; i.e., the portion that is adjacent the device at the time a measurement is taken. This is not easily accomplished, however. Ideally, it would be desirable for the vertical resolution of the device to be sharpened such that the apparatus will respond to formation material only over a vertical interval of very narrow extent. This would enable relatively thin beds to be more readily distinguished and their characteristics to be more accurately measured. At the same time, it is often desirable for the logging device to have a reasonably extensive capability of investigation in a horizontal or radial direction. Not surprisingly, the two considerations are found to be countervailing.
Consider, as an example, the type of well logging apparatus known as an "induction logging" device which includes, inter alia, transmitter and receiver coils mounted in spaced relation on a sonde. Alternating current is applied to the transmitter coil and the alternating magnetic field thereby created generates eddy currents in the formations surrounding the borehole. The eddy currents, in turn, create a secondary magnetic field which induces an electromotive force in the receiver coil. The intensity of the eddy currents is a function of the formation conductivity which can therefore be measured by monitoring the induced electromotive force. The basic principles of induction logging are described in an article by H. G. Doll entitled "Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil Base Mud" which appeared in the June 1949 issue of the Petroleum Transactions of the AIME. Among the techniques which have been devised for sharpening (or "focusing") the vertical response of induction logging devices is to provide additional coils on the sonde. These "focusing coils" are utilized in phased relationship with the basic transmitter and receiver coils to effectively cancel portions of the device response which are above and below the central investigative region of the device. Unfortunately, the use of numerous additional coils in this manner tends to significantly decrease the horizontal range of the device, so there is a practical limit on the degree of such focusing which can be employed. Also, the required additional elements render the device more complex and expensive.
In addition to focusing methods which relate to the overall response characteristic of the downhole device, there have been previously developed vertical resolution improvement methods which are termed "computed focusing" techniques. Generally speaking, computed focusing techniques utilize signals which are, at a given instant, more representative of the formation material in the undesired regions to adjust or correct the signal which is, at the same instant, more representative of the desired region. Computed focusing techniques are described, for example, in U.S. Pat. No. 3,166,709 of H. G. Doll, U.S. Pat. No. 3,230,445 of W. J Sloughter et al, and U.S. Pat. No. 3,457,496 of N. Schuster. These patents describe systems that receive signals from a downhole sensing device and generate computed signals which approximate the response that would have been obtained from a sensing device having a sharper response characteristic. Each of the referenced patents discloses a relatively uncomplicated analog apparatus which can be utilized at a wellsite to produce an on-the-spot log. The apparatus temporarily stores signals obtained at various vertically spaced levels in a borehole and then combines the stored signals in appropriate manner to obtain effective log readings with improved vertical resolution.
While the signal processing techniques disclosed in the referenced patents provide useful results, the logs produced with these techniques nonetheless represent subsurface readings taken with a device having an effective response of substantial (though improved) vertical extent. To visualize the actual subsurface characteristics which, when measured, had produced the log, one must still take into account this effective response. Accordingly, it is one of the objects of the present invention to provide a method for generating a subsurface characteristic model which is consistent with readings taken with a device having an effective response of substantial vertical extent.